Just to make sure this isn't overlooked, the speedo pinion housing is
marked and offset(eccentric) to accommodate the different diameters
needed for the various tooth counts. Make certain you have it indexed
properly for the one You're using!

The charts I have seen all look at diameter/tire height, not the numerical tire sizing..... what matters is the actual distance would be the measurement from the exact center of the hub/wheel assembly to the ground.....but suince that is tough to do right, I figured that taking as accurate a measurement from the ground to the top of the tire would get me close enough.

I have seen when shopping for tires that some of the same tire sizes (for example - a 235 60r 15) have different spec'd tire diameters. Just bought a set of 235 60r 15s for our 80 Volare and found that some brands said one thing and another was a half inch different....and there is a difference between what a targeted size should be versus what it actually is when installed and rolling.....so I would never rely on the advertised tire height to do this calculation.

When I re-geared my Barracuda some time back I just used the diameter of the new tire as mounted, sitting on the ground, and used the gear ratio and found the right number of teeth on the chart,.....worked out well.

Worked for me..... about to do the same for the Volare.....

Keep in mind the the chart has overlap to allow for slight difference in diameter ...for instance a 25.1 and 25.6" tire both use a 28 tooth gear with 2.76s.

what matters is the actual distance would be the measurement from the exact center of the hub/wheel assembly to the ground

Yes...

Quote:

when shopping for tires that some of the same tire sizes (for example - a 235 60r 15) have different spec'd tire diameters

Yes, but only a little bit different, and...

Quote:

overlap to allow for slight difference in diameter ...for instance a 25.1 and 25.6" tire both use a 28 tooth gear with 2.76s.

...this is what makes it easiest to just do the calculation based on the numerical tire size. You might be off by a tenth or two, but it doesn't matter because it's going to be less than the difference between two adjacent pinions. Let's use your numbers, a 25.1" versus a 25.6" tire: that's a 2% difference. 28- vs. 29-tooth pinion, that's a 3.5% difference. Or 28 vs. 27-tooth pinion, that's a 3.7% difference. What makes it best to use the numerical tire size is it's very difficult to accurately measure the diameter of the tire by eye with a yardstick or tape measure.

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